Control apparatus



July 16, 1946. w. H. @ILLE 2,403,917

CONTROL APPARATUS Filed April 20, 1942 5 Sheets-Sheet 1 5\ 54 AMPLmR BRWGE ANI) cmLm-r Powaw.

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July 169 1946. w. H. GILLE 2,403,917

CONTROL APPARATUS Filed April zo, 1942 s sheets-sheet 2 ANO mmm POWER UN\T July 16, 194:6o w. H. GILLE 2,403,917

CONTROL APPARATUS Filed April 2o 1942 s sheets-sheet a Patented July 16, 1946 2,403,917 n CONTROL APPARATUS Willis H. Gille, St. Paul, Minn., assignor to Minneapolis-Honeywell Regulator Company, Minneapolis, Minn., a corporation of Delaware Application April 20, 1942, Serial No. 439,673

19 Claims.

The present invention relates to temperature control systems, and more particularly to temperature control systems for use on aircraft.

The construction of a satifactory temperature control system for use on aircraft presents many unusual problems. For example, the space whose temperature is to be controlled is relatively small, its walls are quite thin, and it is subjected to rapid variations in outside temperature. If an even temperature is to be maintained within the aircraft, it is therefore necessary that a temperature responsive element be employed which respondsrapidly to changes in temperature. Furthermore, it is necessary that the system controlled by the temperature responsive element also be capable of quick response. The required rapidity of response must be much greater than that required in buildings and vehicles operated on the ground, because the temperature changes encountered are much more rapid.

In addition to the requirement of high sensitivity, a practical temperature control system for use on aircraft must be insensitive to the changes in barometric pressure which occur with changes in altitude. It must also be insensitive to vibration, both of the high frequency type sometimes resulting from the operation f the aircraft engine, and the rough irregular bouncing encountered in rough air conditions. An aircraft temperature control system must also be light in Weight and must produce no objectionable radio interference.

It is therefore an object of the present invention to provide an improved temperature control system for use on aircraft. A further object of the present invention is to provide a temperature control system which shall be very sensitive to temperature changes, While remaining insensitive to atmospheric pressure changes and to vibration.

Another object of the present invention is to provide an aircraft temperature control system which is light in weight and which does not produce objectionable radio interference.

Another object is to construct an automatic temperature control system which may be readily installed on existing aircraft to operate existing temperature changes devices of various types which are at present usually controlled manually.

A further object of the present invention is to provide an improved electrical control system for a hydraulic servo-motor. A still further object is to provide an improved control system for a rotary electrical motor adapted to operate valves controlling a hydraulic motor.

Another object of the present invention is to provide a system for controlling the temperature of an aircraft cabin. A further object is to provide, in such a control system, means for regulating the fuel supply to a heater.

Another object of the present invention is to provide an improved system for regulating the temperature of an aircraft engine.

Other objects and advantages of the present invention will become apparent from a consideration of the appended specification, claims, and drawings, in which Figure 1 represents a, plan view of a section of an aircraft, showing the installation of a cabin temperature control system embodying my invention in a somewhat diagrammatic manner,

Figure 2 is a 'circuit diagram showing in greater detail the connections of the bridge, amplier, motor and power supply units used in the system disclosed in Figure 1,

Figure 3 shows, somewhat diagrammatically, the application of vmy invention to a different type of aircraft cabin temperature control system.

Figure 4 illustrates somewhat diagrammatlcally the application of my invention to an aircraft engine temperature control system employing a hydraulic servo-motor.

Figure 5 shows a modified form of hydraulic motor which may be utilized in the system shown in Figure 4,and

Figure 6 shows a diierent embodiment of my invention as applied to an aircraft engine temperature control system.

Referring to Figure 1, there is shown a plan View of a portion of an airplane. The view is taken looking downward at the floor of a companionway I0 between a passenger cabin II and a. Vpilots compartment (not shown). A wall I2, provided with a suitable door I3, separates the companionway from the passenger cabin. Walls I4 and I5 at the sides of the companionway I0 separate the latter from spaces IB and Il, which may, for example, be used for cargo or baggage. Portions of the floor are shown in the drawings as broken away in order to better illustrate the temperature control systems, of which the chief parts are mounted under the floor.

An air intake duct 20, which may be connected with a suitable air intake (not shown) near the front of the plane, is divided by a wall 2 I, into a pair of parallel ducts 22 and 23. Air flowing through the duct 23 passes over a radiator 24,` which may be supplied with a. suitable heating 56 fluid, for example steam, through supply and re- 3 turn pipes 25 and 2l. The duct 22 is provided to by-pass fresh air around the heater 24. The parallel ducts 22 and 2l rejoin each other at 21 and are connected to a suitable distributing duct 2l in the cabin Il.

'I'he relative amounts of air passing through the ducts 22 and 2l are controlled by a set of mixing dampers 30, suitably attached to an operating bar 3|. The bar 3| is attached to a crank arm 32 driven by a motor mechanism 33. The motor mechanism 33 includes a split-phase motor I4, and a gear train (not shown).

The split-phase motor 34 includes a pair of windings 35 and 35, which are spaced 90 electrical degrees apart. in accordance with the conventional construction of such motors. A condenser 31 is connected in parallel with winding 36. The common terminal oi' windings 35 and 38 is grounded as at 4I.

The capacitance of condenser 31 and the inductance of winding 3l are so proportioned that the two form a series resonant loop circuit. This loop circuit is supplied with energy by transformer action from winding 35, but the current flow in winding 36 caused by this transformer action is not sumcient, or of the proper phase relationship, to cause rotation of motor 34. When a slight additional amount of energy is supplied to winding J8, however, it is energized sufficiently to start rotation of motor I4 in a direction determined by the phase of the energy supplied by the amplifier.

Motor 34 is supplied with electrical energy from an amplifier and power unit generally indicated at 4|. This unit has input terminals 42 and 43 and output terminals 44 and 45. Power is supplied to the amplifier and power unit 4| from a battery 46 through power supply terminals 41 and 48. The amplifier and power unit 4| also has terminals 50 and 5| through which electrical energy is conducted to input terminals 52 and 53 of a bridge circuit generally ndicated at 54.

The bridge circuit 54 includes a first temperature responsive element 55. which may be an electrical resistance element having an appreciable temperature coefncient of resistance. The resistance element 55 is shown diagrammatically in Figure l, and may be mounted on the wall I2. 'I'he particular form oi' the temperature responsive resistance element and the means for mounting it on the wall |2 may preferably be as described in the copending application of Russell H. Whempner, Serial No. 439,679, dated April 20, 1942. The resistance element 55 is connected to bridge circuit 54 by a'Dair of conductors 55 and 51.

A second temperature responsive resistance element is mounted in the discharge duct 28 and is connected to the bridge circuit 54 by means of conductors 6I and 52. A third temperature responsive resistance element 63 is mounted in the intake duct 20 and is connected to bridge circuit 54 by means of conductors 54 and 55.

It has been found desirable that the temperature responsive element 55 should have the largest eifect on the system, while the elements 50 and 53 should have smaller effects. The ratio between the effects of these elenents on the system may be controlled by properly designing their relative resistances. For example, in one system built in accordance with the present invention, the cabin temperature responsive element had a resistance of 500 ohms at 70 F., while the compensating resistance elements had a resistance of about 25 ohms at the same temperature.

of the cabin temperature responsive element and that of the outside temperature compensating element. It has been found, however, that the ratio should be at least ten to one in any case, and may be as high as fifty to one.

'I'he bridge circuit 54 is provided with output terminals B5 and 61 connected to the input terminals 42 and 43 of the amplifier and power unit 4|, respectively.

The bridge circuit 54 also includes a rebalanclng potentiometer generally indicated at 10, having a slide wire resistance 1| and a slider 12 cooperating therewith. Slider 12 is connected to the bridge circuit through a conductor 13, while the end terminals of slide wire 1| are connected to the bridge circuit 54 through conductors 14 and 15.

Referring now to Figure 2, it will be seen that the upper left arm of bridge circuit 54 connects input terminal 52 with output terminal 4G and includes a manually adjustable resistance 80, con ductor 51, temperature responsive resistance elcment 55, and conductor 5B. The upper right arm of bridge circuit 54 -connects output terminal $0 with input terminal 53 and includes a conductor 5|, a xed resistance 82, and a conductor 83. The lower left arm of bridge circuit 54 connects input terminal 52 and the slider 12, and includes a conductor B4, a fixed resistance 85. conductor 14, and that part of slide wire 1| between its left hand terminal and the slider 12. The lower right hand arm of bridge circuit 54 connects slider 12 and input terminal 53, and includes that portion of slide wire 1| between slider 12 and its right hand terminal,l conductor 15, a fixed resistance 86, and temperature responsive resistance elements 60 and 63.

A variable resistance 81 is connected in parallel with slide wire 1|. The function of resistance 81 is to determine the amount of movement of slider 12 required to correct a given unbalance of the bridge circuit 54.

The function of adjustable resistance is to determine the value of temperature adjacent the resistance 55 at which the bridge 54 will be balanced. In other words, it operates to set the control point of the system.

The amplifier and power unit 4| includes an amplifier generally indicated at 90, which may be of any type well known in the art in which the phase of the output voltage is reversible with a reversal of the phase of the input voltage. A typical amplifier of this type is shown in the Beers Patent 2,020,275. Another desirable type of amplifler for this purpose is one of the type disclosed in Figure 1 of the copending application of Albert P. Upton, Serial No. 437,561, dated April 3, 1942. Amplifier has input terminals 42 and 43 and output terminals 9| and 92. Bridge output terminal 6B is connected to amplifier input terminal 42 by a conductor B8. Bridge output terminal 61 is connected lto amplifier input terminal 42 by a conductor 69. Input terminal 43 is grounded at 93, and also serves as an output terminal of the amplifier 90. It will be readily understood that the conductor 69 may be omitted,

\ and bridge output terminal 61 connected to ground instead.

While the amplifier referred to in the above mentioned Upton application is one which has been found to be particularly desirable for use in the present apparatus, it is possible to employ any amplifier capable of supplying at the output terminals a voltage which reverses in phase with reversal of the phase of the out-put voltage of the bridge. Such amplifiers are well known in the prior art. It is accordingly not considered necessary to completely illustrate the details thereof. However in order to facilitate an understanding of the general operation of the amplifier, the final amplifier stage is indicated in the drawing. This stage is provided by a double triode tube 90a having two anodes 90b and 90e. A double cathode 9 la is associated with both anodes 90b and 90C. Associated with anode 90b is a grid 92h and with anode 90e is a grid 92e. 'I'he cathode Sla is connected by a lead 91 to the input terminal 43 which in turn is connected to ground 93 as previously noted. 'I'he two grids 92h and 92e are connected together. The junction of the connection to these two grids is indicated by the reference numeral 94a. This junction is shown as being connected by a dotted line connection 95a to the input terminal 42. This connection is shown in dotted lines because in actual practice two or more stages of amplification would be provided between the input terminals 42 and 43 and the grids 92h and 92o. As far as the essential operation of the amplifier is concerned, however, it may be considered as though grids 92h and 92e are directly connected to the input terminal 42 and that the cathode 96 is directly connected by conductor 91 to terminal 43 and hence to ground.

Power is supplied to amplifier '90, bridge circuit 54, and motor 36 from battery 46 through an inverter 94 which may be of the well known vibrator type. Inverter 94 is connected to a primary winding 95 of a transformer 96 having secondary windings |00, |01, |02 and |03.

Secondary winding supplies power to amplifier 90 through conductors |01 and |08.

Secondary winding |0| supplies power to amplier 90 and to winding 36 of motor 34. A connection may be traced from the upper terminal of transformer winding |0| through a conductor ||0 to amplifier output terminal 9|. Another connection may be traced from the lower terminal of secondary winding |0| through a conductor to amplifier output terminal 92. A third connection may be traced from the midpoint of secondary winding |0| through a conductor ||2, terminal 45, a conductor I3, winding 36 and condenser 31 in parallel, ground connection 40, and ground connection 93 to amplifier terminal 43.

Transformer secondary winding |02 supplies power t0 winding 35 of motor 34 through a circuit which may be traced from the lower terminal of winding |02 through a conductor ||4, a condenser H5, a conductor ||6, terminal 44, a conductor I1, winding 35, ground connection 40, a ground connection |20, and a conductor |2| to the upper terminal of secondary winding |02.

Secondary winding |03 is connected to terminals 50 and 5I, which are connected to input terminals 52 and 53 of bridge 54 through conductors |05 and |06 respectively.

Operation of Figures 1 and 2 When the parts are in the positions shown in the drawings, both sets of dampers 30 are half wire 1|. Let it be assumed that the bridge 54 is balanced, and that the adjustable resistance has been set so that the bridge remains balanced when the temperature in the cabin is 70.

When the bridge circuit 54 is balanced, the output terminals 66 and 61 are at the same potential, so that no signal is applied to the input terminals of amplifier 90. Under these conditions, no alternating current flows through winding 36 of motor 34. Although winding 35 of motor 34 is continuously energized by its connections to transformer secondary |02, both windings 35 and 36 must be energized with currents of the proper phase relationship in order to cause rotation of motor 34, in accordance with the well known characteristics of split phase motors. Condenser ||5 is connected in series with motor winding 35 in order to shift the phase of the current supplied to that winding approximately electrical degrees with respect to the potential and the terminals of transformer secondary winding |02.

When the bridge circuit 54 is balanced, the output terminals 66 and 61 are at the same potential, which is at some value intermediate the potentials of input terminals 52 and 53. If, after the bridge is balanced, the resistance of element 55 ldecreases, the potential of output terminal 66 assumes a new value closer to that of input terminal 52. Since the output terminal 61 retains its original potential value, it will be seen that there is a potential difference between output terminals 66 and 61. As between the two output terminals, the potential of output terminal 66 is closer to that of input terminal 52, while the potential of output terminal 61 is closer to that of input terminal 53. Therefore, the unbalance potential appearing between output terminals 66 and 61 is in phase with the potential supplied to input terminals 52 and 53. 0n the other hand, if the resistance of element 55 increases after the bridge is balanced, the potential of output terminal 66 assumes a value closer to that of input terminal 53. The output terminal 61 retains its original potential value, and therefore a potential difference appears between output terminals 66 and 61. In this case, however, as between the two output terminals, the potential of terminal 66 is closer to that of input terminal 53, While the potential of terminal 61 is closer to that of input terminal 52. Therefore, it may be seen that the unbalance potential appearing between output terminals 66 and 61 is opposite in phase to that supplied to input terminals 52 and 53.

Consider now the effects upon the balance of the bridge circuit of changes in the resistance of elements 60 and 63. If, after the bridge is balanced, the resistance of either element 60 ci 63 decreases, the output terminal 61 thereby assumes a potential closer to that of input terminal 53. The potential of output terminal 66 retains its original value, so that, as between the two output terminals, the potential of terminal 66 is closer to that of input terminal 52, while the potential of terminal 61 is closer to that of output terminal 53. Therefore, the unbalance potential appearing between output terminals 66 and 61 is in phase with the potential supplied to input terminals 52 and 53.

On the other hand. if the resistance of either element 60 or 63 increases after the bridge is balanced. the output terminal 61 thereby assumes a potential closer to that oi' input terminal 52.

The potential o! output terminal 8B retains its original value, so that, as between the` two cutput terminals, the potential of terminal EI is closer to that of input terminals 53, while that of terminal I1 is closer to that of input terminal l2. Therefore, the imbalance potential appearin! between output terminals .I and B1 is opposite in phase to that supplied to input terminals I2 and B3.

Recapitulating, it will lbe seen that a decrease in the resistance oi' any of the elements il, SII or cause an alternating potential to appear at output terminals and l1, which is in 'phase with the potential supplied to input terminals l2 and BI. 0n the other hand, an increase in resistance oi' any of the elements Il, Il or Il will cause an alternating potential to appear at output terminals It and 61 which is opposite in phase to that of the potential supplied to input terminals l! and B3.

A rise in temperature either in the intake d-uct 2l adjacent resistance element C3,l in the discharge duct 28 adjacent the resistance element Il, or in the cabin II itself adjacent resistance element I! indicates that less heat is necessary to maintainthe cabin II at the desiredtemperature. When such a rise in one of the three temperatures referred to occurs, the resistance of the corresponding one of the temperature responsive elements t5, 6I and I3 increases, thereby unbalancing the bridge circuit 5l in such a direction that an alternating potential appears. at output terminals 88 and 61 having a phase' opposite to that of the potential supplied to terminals 52 and 53. On the other hand, when a decrease in any of these temperatures occurs, a need for the supply oi' additional heat to the cabin II is indicated, and the bridge is uxrbalanced in such a direction that a potential appears between terminals 6B and 61 having the same phase as the power supplied to input terminals I2 and B3.

Two possible circuits may be traced through the motor neld winding 3i. One of these circuits extends from the upper terminal of secondary III through conductor III, terminal DI, anode IIb. cathode sla, conductor 91, ground connections 93 and l0, neld winding 3B, conductor Ill, terminal l5 and conductor H2 to the center tap o! secondary IIII. The other circuit extends from the lower terminal of secondary Ill through conductor III, terminal 92. anode 80c, cathode lla, conductor l1, ground connections 93 and ll, ileld winding Il, conductor III, terminal ll, and conductor Il! to the center tap of secondary IIII. Current will ilow through the iirst traced circuit only during the half cycle in which the anode IIb is positive with respect to the cathode. This will occur only during the half cycle when the upper end of the secondary IOI is positive with respect to the lower. Current will iiow through the last mentioned circuit, on` the other hand, only when anode lic is positive with respect to the cathode and hence the lower end of secondary III is positive with respect to the upper end. Thus It is possible only during alternating hall' cycles for current to ilow through the respective 4 circuits traced. It is furthermore possible for current to ilow through either one of these current paths only when the eilect oi the amplifier signal potential as applied to the grid is to tend to raise the potential of the grid during the halt cycle in which the anode is positive. Since, as was previously noted, the grids' B2b and 92e are connected together. it will be readily apparent that the voltages applied to these two grids are identical and hence .in phase with each other. The voltages applied to the two anodes llb and "c are out of phase with each other. As a result, the amplifier signal voltage applied to the grids B2b and 82e will be in phase with the anode voltage applied to either one or the other oi' the two anodes so that current will ilow through one or the other of the two paths traced. The phase of the signal volta-ge is dependent upon the phase of the output voltage oi' the bridge which, as previously explained, is dependent upon the direction in which the bridge is unbalanced. Thus, one or the other o i' the two circuits traced is rendered effective depending upon the unbalance oi the bridge. Since the voltages applied to these two circuits are displaced in phase with respect to each other, it will be obvious that the current ilowing through winding Il when the bridge is unbalanced in one direction will be 180 displaced with respect to that ilowing through the same winding when the bridge is unbalanced in the opposite direction. Since the output voltage of secondary III is in phase with the output voitage oi.' secondary IBI which supplies the input voltage to the bridge, it will be readily apparent that the current owing in winding il will be oi the same or opposite phase as the terminal voltage of the secondary winding |02.

Since the current ilowing in winding Il is of the same or opposite phase as the terminal voltage of the transformer secondary winding, while the current flowing in winding 35 is shifted 90 degrees in phase in respect to the terminal voltage of the transformer secondary winding, motor Il will be driven in one direction or the other depending upon whether the current in winding ll is of the same or the opposite phase asthat of the secondary terminal voltage.

The connections between the bridge circuit 5l and the amplifier 90, and between amplifier 8l and motor Il, are such that when the bridge is unbalanced in a direction indicating the need for less heat in the cabin II, motor 36 is driven in a direction to close the dampers in the heater duct 2l and open the dempers in the by-pass duct 22, thereby supplying less heat to the cabin II. This operation of motor Il also drives slider 'l2 to the right along slide wire 'II, thereby rebalancing the bridge circuit Il.

When the temperature adjacent either of the three temperature controlling resistance elements Il, Il and 83 decreases. the bridge is unbalanced in such a direction that amplifier l0 responds to drive motor I4 so as to close the dampers in bypass duct 22 and open the dampers4 in heater duct 2l, thereby supplying more heat to the cabin II. This is done by driving lever arm 3| and slider 'l2 to the left. Motion oi slider 'I2 to the leit rebalances the bridge circuit so as to stop the motor Il.

It should be noted that in the amplifier and power unit 4I, the only wound coils or electromagnetic devices using iron cores are those associated with inverter 94, transformer 98, and motor Il. The weight oi the amplifier and power unit has thereby been maintained at a minimum. Furthermore, there are no switch contacts in the entire system, except those associated with inverter Il, which may be readily shielded so as to produce no radio interference.

Figure 3 In Figure 3 is disclosed an aircraft cabin temperature control system in which the control is accomplished by modulating the fuel supply to a uid fuel burning heater arrangement. In this ligure, all parts of the system which correspond exactly to similar parts in Figure 1 have been given the identical reference numerals. It will be noted that the bridge circuit 54, the amplier and power unit 4|, the motor mechanism 33, are the same asin Figure 1. Furthermore, the location of the temperature responsive resistance 63 in the intake duct 20, the location of temperature responsive resistance 60 in the discharge duct 28, and the location of temperature responsive resistance 55 in the cabin I I are the same as the location of the corresponding elements in Figure 1.

In Figure 3, the air supplied to the cabin passes through the intake duct 20, over a plurality of fluid fuel burning heaters |20, and through the discharge duct 28 to the cabin Fuel is supplied to the heaters |20 through a fuel supply pipe |2|. The supply of fuel to the heaters |20 is controlled by a modulating valve |22, and by a shut-off valve |23.' Any suitable source of iluid fuel may be used. For example, in certain installations, the fuel has been a combustible mixture of gasoline and air drawn from the intake manifold of one of the aircraft engines.

The modulating valve |22 is operated by the motor mechanism 33. The valve |22 has an opening |24 in the center thereof to insure that it never fully closes, but always permits a minimum flow of fuel therethrough.

The shut olf valve |23 is provided with an electricai operator |25, which may be a solenoid or a rotatory motor, for example. Each of the heaters |20 is provided with a suitable ignition mech- 'anism such as the hot wire igniter shown somewhat diagrammatically at |26. Energization of valve operator |25 and the igniters |26 is controlled by a switch 21 comprising a stationary contact |30 and a movable switch arm |3|. The switch arm |3| carries a follower |32 of insulating material, cooperating with a cam |33 driven by the motor mechanism 33.

Operation of Figure 3 For purposes of clarity, the modulating valve |22 has been shown in Figure 3 in its fully open position. The slider 12 of rebalancing potentiometer is shown at its corresponding position at the upper extremity of slide wire 1|. Likewise, the cam |33 is shown in a position wherein it has closed switch |21, thereby completing energizing circuits for shut-off valve operator |25 and igniters |26.

The energizing circuit for operator |25 may be traced from the upper terminal of battery 46 through a conductor |34, switch |21, a conductor |35, a conductor |36, operator |25, and ground connections |31 and |38 to the lower terminal of battery 46. The energizing circuit for igniters |46 may be traced from the upper terminal of battery46 through conductor |34, switch |21, conductors I 35 and |40, and a plurality of parallel conductors |4| to the respective igniters |26,

only one of which is shown, the frame of burners ply of fuel to the burners |20 is maintained, and

the fuel is ignited by the igniters |26.

As in Figure 1 the motor mechanism 33 modulated the position of the damper 30 in response to temperature changes, so in Figure 3 the motor mechanism 33 modulates the position of the valve |22. It is believed that this operation of valve |22 by motor mechanism 33 will be readily understood from what has gone before, and need not be further described.

As the valve |22 moves towards its closed 'position from the position shown, the cam |33 is ro.. tated in a counter-clockwise direction. The construction is such that when the valve |22 reaches its minimum position, the follower |32 of switch arm |3| has nearly reached the end of the high dwell portion of cam |33. Ii' the bridge circuit 54 then becomes unbalanced so as to indicate a need for further reduction in the supply of fuel to the heaters |20, cam |33 rotates a slight additional amount in a counter-clockwise direction. This moves the high dwell portion out from under follower |32, allowing it to drop to the low dwell portion, thereby opening switch |21. Opening of switch |21 shuts out the supply of electrical energy to the operator |25 and to the igniters |26. Deenergization of operator |25 causes shut oil' valve |23 to close.

It should therefore be apparent that the supply of fuel to the heaters |20 is modulated from a maximum to a minimum value and that if a further reduction in the supply of fuel is necessary, the supply is cut off completely and the igniters are deenergized. In heaters using highly combustible fuels of the type described, it is necessary that a minimum supply of fuel be maintained in order to prevent a flash back, a phenomenon in which the flame moves back along the fuel supply pipe to the source of fuel. This minimum supply is maintained by the system shown, until the system is completely shut down.

Figure 4 In Figure 4 is shown an engine temperature control system embodying my invention. An aircraft engine |50, of the air cooled type. is provided for driving a propeller |5|. The engine |50 has a. cowl |52, of conventional form. Air for cooling the engine |50 flows through an opening at the front of the engine, which is not shown in detail in the drawings, but Whose general location is shown at |53. After passing over the engine, the heated air passes out through nap |54 in the cowl |52.

A temperature responsive device |55 is mounted on the engine |50. It may, for example, be conveniently attached to a spark plug |59. Resistance element |55 is connected in a. bridge circuit generally indicated at |56, having input terminals |51 and |58, and output terminals |60 and |6|.

Input terminals |51 and |58 are connected through conductors |62 and |63 into terminals 50 and 5| respectively of an amplifier and power unit 4|, similar to the corresponding amplifier and power unit shown in Figures l. 2, 3. Output terminal |60 of bridge |56 is connected through a conductor |64 to input terminal 42 of amplifier and power unit 4|. Output terminal |6| of bridge |56 is grounded at |65, and is therefore connected through ground connections |65 and 03 to input terminal 43 of amplifier and power unit 4|.

The upper left hand arm of bridge circuit |56 connects input terminal |51 with output terminal |60, and includes a conductor |66, resistance element |55, conductors |61 and |66, and' that portion of a slide wire resistance |10, between its left hand terminal and its cooperating slider |1|,

ll which is electrically connected to output terminal |00. Slide wire and slider |1| constitute-a rebalancing potentiometer |12, hereinafter referred to as the ultimate rebalancing potentiometer.

The upper right arm of bridge circuit |58 connects output terminal |80 and input terminal |58, and includes that portion of slide wire |10 between slider |1| and its right hand terminal, a conductor |18, and a fixed resistance |14. An adjustable resistance is connected in parallel with slide wire |10 in order to determine the amount oi movement oi' slider |1| necessary to correct a given imbalance oi the bridge circuit I 58.

The lower left arm of bridge circuit |58 connects input terminal 51 with output terminal I 8 I and includes a conductor |18, a iixed resistance |11, and that part of a slide wire resistance |80 between its left hand terminal and its cooperat- 1118 slider |8|. Slide wire |80 and slider |8| iorm a rebalancing potentiometer hereinafter referred to as the proximate rebalancing potentiometer.

The lower right arm of bridge circuit |58 connects output terminal |8| with input terminal |58 and includes that portion of slide wire |80 ybetween slider |8| and its right hand terminal, a conductm' |88, a i'ixed resistance |84, and a variable resistance |85. Ihe variable resistance |85 is provided in order to adjust the temperature at which the control system maintains the engine |50. A variable resistance |88 is connected in parallel with slide wire |80 in order to determine the amount of movement oi' slider |8| necessary to correct a given unbalance of bridge |58.

Motor mechanism 88. which is controlled in response to the unbalance potential of bridge |58 acting through amplifier and power unit 4|, drives, through a gear mechanism schematically indicated at |81, a shaft |88 connected to slider |8|, and a shaft |90 which carries a series of cams |8|, |82, |98 and |84.

The cams |8|, |82, |88 and |84 operate a series of four poppet valves |85, |88, |91 and |98, respectively. These poppet valves control the ilow of high pressure iluid to a hydraulic servo motor 280, of conventional design.

A suitable source o! high pressure iluid is provided for operating servo motor 200. this source being shown in the drawing as a pump and high pressure chamber associated therewith and numbered High pressure fluid may rlow from the high pressure chamber 28| through a suitable conduit 282 to an inlet chamber 202 in a valve casing 204, which contains the poppet valves |95, |88, |81 and |88. Valves 88 and |81 control the i'low of high pressure fluid from inlet chamber 288 to valve chambers 288 and 208, respectively. Valves |85 and |88 control the iiow of iiuid from valve chambers 205 and 208 to outlet chambers 201 and 208, respectively. Outlet chambers 201 and 208 are connected by conduits 209 and 2|0 respectively, to a sump or receiver of low pressure iiuid 2| The servo motor 208 comprises a cylinder 2|2 having a piston 2|8 reciprocatable therein. One end of the cylinder 2|2 is connected through a conduit 2|4 to the valve chamber 208, and the other end oi' the cylinder 2 I2 is connected through a conduit 2|5 to the valve chamber 205.

A piston rod 2|8 is attached tc the piston 2|8 and passes through a suitable gland, not shown, in the end of cylinder 2|2, and through a bearing 2|1, of any suitable construction, which may be attached to a iixed part of the aircraft. Pivotally suitable part of its pivotal connection with slider |1| may .be made of insulating material in order to prevent grounding oi! the slider |1|.

Operation of Figure 4 When the parts are in the position shown in the drawing, the ilaps |54 are half-way open, and the piston 2 I3 is in a central position in the cylinder 2| 2. The bridge circuit |58 is balanced, the sliders |1| and |8| are in their center positions, and all the valves |9|, |92, |93 and |84 are closed. Under these conditions, let is be assumed that the ytemperature of the engine |50 increases, thereby increasing the resistance of sensitive element |55, thus increasing the resistance between input terminal |51 and output terminal |1| of bridge |58. This unbalance of bridge circuit |56 causes a signal to be impressed on the amplifier and power unit 4|, which in turn transmits a control impulse to the motor mechanism 33. The connections are so arranged that the control impulse transmitted to motor 33 is in the proper direction to cause it to rotate shaft in a clockwise direction, and also to rotate shaft |88 in a clockwise direction. Rotation oi shaft |88 clockwise causes movement of slider |8| to the right along slide wire |80, thereby producing an increase in the resistance between input terminal |51 and output terminal |8| to balance the increase between input terminal |51 and output terminal |80 caused by the increase in engine temperature. This rebalancing of the bridge circuit |58 causes motor 33 to stop substantially as soon as the valves are opened.

Rotation of shaft |90 in a clockwise direction causes opening of valves and |91 under the influence of cams 8| and |93. Valves |98 and |98 remain closed. High pressure uid is then admitted to the right hand end of cylinder 2|2 from inlet chamber 203 through valve |91, valve chamber 208, and conduit 2 |4. At the same time, the pressure on the iluid in the left hand end of cylinder 2| 2 is released through conduit 2 5, valve chamber 205, valve |85. and outlet chamber 201. 'Therefore a pressure differential is established between the opposite faces of piston 2|8, thereby forcing it to the left from the position shown in the drawings. Movement of piston 2|3 and its associated piston rod 2|8 to the left causes the links 22| and 222 to push the cowl aps |54 further open, thereby causing an increased flow of air over engine |50 to provide additional cooling thereof, so as to tend to restore its temperature to the value previously determined by the setting of resistance |85.

Movement of piston rod 2|6 to the lei't is also transmitted through links 22| and 223 to the slider |1|. Movement of slider |1| to the left causes a decrease in the resistance between input terminal |51 and output terminal |80. This causes an unbalance of bridge circuit |58 in the opposite direction from that caused by the previous unbalance, so that a control impulse is transmitted to motor mechanism 33 causing it to rotate shafts |88 and |90 in a counter-clockwise direction. Rotation of `shaft |88 counter-clockwise causes slider 8| to be driven to the left across slide wire |80, thereby rebalancing the bridge circuit, while rotation of shaft |90 in a counter-clockwise direction causes closure of valves I9| and |93, thereby stopping operation of servo motor 200.

'Ihe resistance of slide wire |80 is made much smaller than/the resistance of slide wire |10. Because oi' this, the effect of potentiometer |12 on the balance of the bridge circuit |50 is greater than the effect of potentiometer |82. In other words, if the slider I1| moves through a given portion of its range of travel, the slider |8| must move through a greater portion of its range in order to rebalance the bridge-circuit |55. Therefore, a small movement of slider |1| in response to opening of the valves |9| and |93, for example, is sufllcient to cause movement of slider |8| corresponding to the full closing movement of the valves.

Therelative effects of sliders |1| and |8| on the balance of the bridge, and hence the amount of movement of slider |1| required to cause closure of the valves, may be regulated by adjusting the variable resistances and |88.

By proportioning the relative effects of potentiometers |12 and scribed, fullopening and closure of the valves may be controlled by very small unbalancing effects in the bridge circuit. Full power is thereby made available for control purposes even through the correction required is small.

It should be apparent that the rebalancing action to a certain extent without I Figure 5 There is shown in Figure 5` a modified form of hydraulic servo motor which may be used in the system disclosed in Figure 4, and when used therein results in a substantially different mode of operation of the system. The servo motor 225 of Fig. 5 comprises a. cylinder 226 and a loosely tting piston 221 reciprocable therein. The other parts are the same as in Figure 4, as indicated by their reference characters. It will be understood that a tightly iltting piston with a, leak port or any similar' arrangement might be used. so long as there is a small leak between the ends \of the cylinder. The size of the leak as it appears in Figure 5 has been exaggerated for the sake of clarity. In practice, a very small orifice between the ends of the cylinder would be sum- |82 in the manner de- 25 tion of the proximate rebalancing potentiometer |82 is only temporary, and that the slider |8| always nally comes to rest at or near its center position. The ultimate rebalancing of the bridge circuit following an unbalance thereof is accomplished by the slider |1| in accordance with the new position of cowl ilaps |54. When the bridge |58 is ultimately balanced the slider |8| is always in or near its center position and all the valves |9|, |92, |93, and |94 are closed.

When the temperature of the sensitive resistance element |55 drops below the value which maintains the bridge circuit |55 balanced, a signal of the opposite phase is applied to the ampliiler and power unit 4|, thereby causing rotation oi' motor mechanism 33 in the opposite direction so as to rotate shafts |88 and |90 in a counterclockwise direction, thereby moving slider |8| to the left and opening valves |92 and |94. It will be readily understood that this causes movement of piston 2|3 to the right, thereby movingvslider |1| to theright and causing slider .|8| to be returned to its center position as the valves |92 and |94 are closed by rotation of motor 33.

It should be apparent from the shape of cams |9|, |92, |93 and |94 that this system has a considerable dead spot. In other words, there is a considerable angle through which the shaft |90 may rotate without actuating any of the valves |95, |98, |91 and |98. There is of course a corresponding range of movement of slider |8| on either side of its center position in which the bridge circuit may be balanced by the movements oi' this slider without causing movements of the slider Ill. Since it is permissible to allow the cient.

When servo motor 225 is used in the system of Figure 4, the system is completely stable only when the piston 221 is at a position determined by the bias of the load on it. In the present case, the aps |54, due to the balance between the outside and inside air pressures acting on them, always tend to return to a central position, which is approximately the position shown in the drawings. The load on the servo motor may therefore be said to bias it for movement to its center position. When the servo motor 225 is used in the system of Figure 4, and the bridge is balanced, the piston 221 therefore moves slowly toward its center position, the movement being controlled by engine temperature to vary about the desired the flow of uid through the leak or orifice between the ends of the cylinder 225. Therefore the slider 1| also moves slowly to its center position. During this movement the bridge circuit is maintained in balance by the rebalancing action of slider 8|, driven by motor 33.

' When the rebalancing action required of slider |8| carries it to a, point where one pair of valves begins to open, the servo motor piston 221 is given an impulse in the opposite direction away from its center position. This of course causes slider |1| also to move away from its center position and unbalances the bridge in the opposite direction so that motor mechanism 33 again closes the valves which were opened. 'I'he net result is that the position of piston 221 tends to slowly oscillate about a point such that the balance of the bridge is maintained with the slider |8| at one end or the other of its dead spot. The piston 2|3 moves toward its center position just far enough to unbalance the bridge so as to cause one set of poppet valves to open slightly, whereupon the piston is given an impulse in the opposite direction by the high pressureiluid, thereby unbalancing the bridge in the opposite direction and causing closure of the poppet valves. This action is cyclically repeated.

When the system is operating under these conditions, a slow variation in the temperature of the engine |50 will cause a change in the median position about which the piston 221 oscillates. The slider |8| will continue to oscillate about the same end of its dead spot, thereby causing operation of the same setv of valves as long as the rate of change of resistance |55 is lower than the rate of change of resistance |12 to the movement of slider |1|. When the temperature of engine |50 chan-ges rapidly however, resistance |55 also changes rapidly, causing a greater movement of slider |8| to open the valves fully. Ii the change is in the opposite direction, the slider will be moved clearacross its dead spot and will open the opposite set of valves to produce the required control eiect on the cowl flaps |54.

It should be understood that this type of control is not limited to the system described herein, but may be applied to other control systems. In a system where a rapid response is not required under certain conditions, one set of poppet valves could be dispensed with, and the system allowed to oscillate under the control of a single set of poppet valves.

Figure 6 In Figure 6 is shown an aircraft engine temperature control system utilizing a modiiied embodiment of my invention. Figure 6 shows a system for controlling the temperature of an air cooled engine |55 by modulating the position of the cowl naps |54 in a manner generally similar to that disclosed in Figure 4. The system shown in Figure 6 utilizes a temperature responsive resistance element |55 which may be the same as that used in Fig. 4. A different type of bridge circuit is used however, and a somewhat different amplifier and motor control system is also used, the system being completely electrical, rather than partly electrical and partly hydraulic as in Fig. 4.

Temperature responsive resistance element |55 is connected in a bridge circuit 255 having input terminals and 252 and output terminals 255 and 254. The upper left arm of bridge circuit 255 connects input terminals 25| with output terminal 255 and includes temperature responsive resistance element |55. The upper right arm of bridge circuit 255 connects output terminal 255 and input terminal 252, and includes a fixed resistance 255. The lower left arm of bridge circuit 255 connects input terminal 25| and output terminal 254, and includes a xed resistance 255, a conductor 251, and that part of a slide wire resistance 255 between its left hand terminal and its cooperating slider 255. Slide wire 255 and slider 255 constitute a rebalancing potentiometer for the bridge circuit 255.

The lower right arm of bridge circuit 255 connects output terminal 254 with input terminal 252 and includes that part of slide wire 255 between the slider 255 and its right hand terminal, the conductor 25|, and an adjustable resistance 252. The function of resistance 252 is to determine the control point of the system, in order words, it determines that temperature adjacent to resistance |55 which causes the bridge to be balanced when the rebalancing potentiometer 255 is in a i given position. A variable resistance 255 is connected in parallel with the slide wire 255. The function of resistance 255 is to determine the amount of movement of slider 255 necessary to connect a given unbalance of the bridge 255.

Output terminal 255 is connected to an input terminal 255 or an electronic amplifier 255.

\ Amplifier 255 may be of any desired type, but I terminals 215 and 214.

Output terminal 215 isconnected through a conductor .215, winding 218 of relay 211, a conductor 215, transformer secondary winding 255, and a conductor 25| to ground at 252.

Output terminal 214 is connected through a conductor 255, coil 254 of a relay 255, a conductor 255, the lower half of transformer secondary winding 255, and conductor 25| to ground at 252.

Secondary winding 255 forms a part of a transform'er 251 having a primary winding 255 and additional secondary windings 259 and 255.

Secondary winding 255 is connected through conductors 255 and 25|, respectively to input terminals 25| and 252 of bridge circuit 255. The secondary winding 255 is connected through conductors 255 and 254 to power supply terminals 21| and 212 of amplier 255.

Primary winding 255 is supplied with alternating current by means of an inverter 255 of any well known type, which is in turn supplied with direct current from a battery 255 through conductor 251 and 255.

Relay 211 includes a switch arm 555 movable into engagement with a stationary contact upon energization of coil 215. Relay 255 includes a switch arm 552 movable into engagement with a stationary contact 555 upon energization of coil 254.

The switches 555 and 552 control the energization of a reversible direct current motor generally indicated at 554, comprising an armature 555 and a. pair of field windings 555 and 551. The field windings 555 and 551 are so connected with respect to the armature 555 that their selective energization causes rotation of the motor in opposite directions.

Motor 554 drives, through a gear train schematically indicated at 5|5, a pinion III, which cooperates with a rack 5|2 on a thrust rod 5|5.

The thrust rod 5|5 is slideable through a bearing 5|4 of any suitable construction, which may be attached to a fixed part of the air craft. Pivotally attached to the thrust rod 5|! are a pair of links 5|5 and 5|5, pivotally connected at their opposite ends to cowl flaps |54. A motion reversing link 5 |1 is mounted on a fixed pivot, as at 5|5, and has one end attached by a suitable connection such as a pin and slot arrangement, to the thrust rod 5|5. The other end of link 5|1 is pivotally attached to a link 5|5, whose opposite end is attached to slider 255.

Operation of Figure 6 The ampliiier 255 is so constructed that when an alternating signal of a predetermined phase is applied to the input terminals 255 and 215, that branch of the output circuit extending through output terminal 215 and relay 211 is energized. When an alternating current signal of the opposite phase is applied to input terminals 255 and 215, the other branch of the output circuit including terminal 214 and relay 255 is energized. It will therefore be apparent that the relays 211 and 255 are selectively energized in accordance with the direction of unbalance of bridge circuit 255.

With the parts in the positions shown in the drawings, let it Vbe assumed that the temperature adjacent the temperature responsive element |55 increases above the value which it is desired to maintain. This increases the resistance between input terminal 25| and output terminal 255 of 1 bridge circuit 255. This unbalances the bridge 255 in such a direction that an alternating potential is applied to ampliiler 255 with the proper phase relationship to cause energization ofthe relay winding 215. 'Energization of relay winding 213 causes switch arm 300 to move into engagement with contact 30|, thereby completing andriving thrust rod 3|3 to the left. and opening cowl flaps |54. Movement of thrust rod 3l! to the left also acts through links 3|1 and 3|3 to move slider 253 to the right, thereby increasing the ressitance between bridge input terminal 25| and output terminal 254, and balancing the increased resistance between input terminal 25| and output terminal 253 due to the increase in engine temperature. When the bridge is again balanced, the relay 211 is deenergized and motion ol.' the cowl ilap |54 and the slider 259 ceases.

If the engine temperature decreases below the value it is desired to maintain, the bridge 250 is unbalanced in the opposite direction, thereby causing energizati'on of winding 284 of relay 285. Energization of relay winding 284 causes switch arm 302 to be closed against stationary contact 303, thereby completing an energizing circuit for winding 306 and armature 305 of motor 304. This circuit may be traced from the lower terminal of battery 295 through conductor 320, switch arm 302, contact 303, a conductor 325, iield winding 306, brush 322, armature 305, brush-323, and conductor 324 to the upper terminal of battery 296. Completion of this circuit causes rotation of motor 304 in such a direction as to rotate pinion 3|| clockwise thereby driving thrust rod 3I3 to the right and moving the cowl flaps |54 toward closed position. This movement of thrust rod 3|3 to the right also drives slider 259 to the left, thereby rebalancing the bridge circuit 250. I

While I have shown and described certain specific embodiments of my invention, other modiiications thereof will readily occur to those skilled in the art, and I therefore wish to be limited only by the scope of the appended claims.

I claim as my invention:

l. In combination, an aircraft, means deiining a space within said aircraft, duct means iorsupplying air to said space, means for utilizing the motion oi said aircraft to induce a ilow of air through said duct means, heater means in said duct means, means for controlling the amountv of heat transferred by said heater to the air iiowing through said duct means, a temperature responsive resistance element, means for mounting said element in a position exposed to'the temperature of the air in said space, a second temperature responsive resistance element, means for mounting said second element in said duct v means on the up-stream side of said heater means,

ysaid output circuit and said motor means.

2. :in4 combination, an aircraft, means dening a space within said aircraft, duct means for supplying air to said space, means i'or utilizing the motion of the aircraft to induce a iiow oi air through said duct means, a heater mounted in said duct means including a iluid fuel burner, valve means for modulatingly controlling the supply of fuel to said burner, an element having an appreciable temperature coeiiicient oi' resistance, means for mounting said element in a position exposed to the temperature of the air in said space, a normally substantially balanced electrical network including said element, an electronic ampliner having an input circuit and an output circuit, means for rebalancing said network, electrical motor means for operating said valve means and said rebalancing' means simultaneously, a, continuously conductive electrical connection between said network and said input circuit, and another continuously conductive electrical connection between said output circuit and said motor means. i

3. A system for controlling the temperature of an aircraft engine, comprising in combination, means for cooling said engine, an element having an appreciable temperature coeicient of resistance, means for mounting said element in proximity to a part oi said engine, an electronic amplifier having an input circuit and an output cir cuit, electrical motor means for controlling said cooling means, a connection between said element and said input circuit, and a connection between said output circuit and said motor means.

4. A system for controlling the temperature of an aircraft engine, comprising in combination, means for cooling said engine, hydraulic means for operating said cooling means, an element having an appreciable temperature coemcient of resistance, means for mounting said element in proximity to a part of said engine, an electronic amplier having an input circuit and an output circuit, electrical motor means for` controlling said hydraulic means, a connection between said element and said input circuit, and a connection between said output circuit and said motor means.

5. uA system for controlling a iluid operated servo-motor, comprising in combination, means for supplying iluid under pressure, valve means controlling the flow of iluid from said supply means to said servo-motor, a device having an electrical characteristic variable in accordance with a condition indicative of the need for opera.'- tion of said servo-motor, a normally balanced electrical network including said device, said device being operative upon a change in said condition to unbalance said network, means operated by said servo-motor for rebalancing said network, an electronic amplifier having an input circuit and an output circuit, means connecting said device and said input circuit, electric motor means for operating said valve means. and means connecting said electric motor means and said output circuit.

6. A system for controlling a fluid operated servo-motor, comprising in combination, means for supplying iiuid under pressure, valve means controlling the flow of iluid from said supply vmeans to said servo-motor, a device having an electrical characteristic variable in accordance with a condition indicative oi the need for operation of said servo-motor, a normally balanced electrical network including said device, said device being operative upon a change in said condition to unbalance said network, an electronic amplier having an input circuit and an output circuit, means connecting said device and said 19 input circuit, electric motor means for operating said valve means, means connecting said electric motor means'and said output circuit, means op- 1. A control system, comprising in t'z'oxnbiniei` tion. a pressure operated reversible fluid motor. means for supplying fluid under pressure, at least two selectively operable valve means for oo ntrolling the flow of fluid from said supply means to said fluid motor so as to cause operation thereof selectively in either direction, reversible electrical motor means .for operating said valve means, said electrical motor means having\a normal position wherein all of said valve means are closed. and `being operable in opposite directions from said position to selectively open said valve means, a device having an electrical characteristic variable in accordance with a condition indicative of the need for operation o! said fluid motor, a normally -balanced electrical network including said device, said device being operative upon a change in said condition to unbalance said network, means responsive to unbalance of said network to energize said electrical motor means for operation in a direction corf responding to the direction of said unbalance. thereby opening one of said valve means. means driven by said electrical motor means to re- `balance said network. and means driven by said fluid motor to unbalance said network in the opposite sense, thereby energizing said motor i'or` operation in a direction to close said one valve means and again rebalance said network.

8. A control system, comprising in combination, a load device biased to a predetermined position, a pressure operated fluid motor for driving said load device in opposite directions,'

means for supplying fluid under pressure, a pair ot valve means for controlling the flow of fluid from said supply means to said fluid motor so as to cause operation thereof selectively in either direction, leak means in said motor for permitting slow operation of said motor under the lniluence of said biased load device when both said valve means are closed. reversible electrical motor means for operating said valve means, said elecf trical motor means having a normal position wherein both oi said valve means are closed, and being operable in opposite directions from said position to selectively open said valve means, a device having an electrical characteristic variable in accordance with a condition indicative of the need for operation of said fluid motor, a normally balanced electrical network including said device, said device being operative upon a change in said condition to unbalance said network, means responsive to unbalance of said network to energize said electrical motor means for operation in a direction corresponding to the direction of said unbalance, thereby opening one of said valve means. means driven by said electrical motormeans to rebalance said network, and means driven by said iluid motor to change the balance condition oi' said network in the same sense as said rebalancing means, so as to cause reversal of said electrical motor.

9. A control system, comprising in combination, a load device biased to a predetermined position, a pressure operated fluid motor for driving said load against its bias, means for supplying fluid under pressure, valve means ior controlling the flow of iluid from said supply` means to said fluid motor, leak means in said motor for permitting slow operation ot said motor under the influence of said biased load device whenl said valve means is closed, a device having an electrical characteristic variable in accordance with a condition indicative of the need ior operation of said load device, a normally balanced electrical network including said device, said device being operative upon a change in said condi.. tion to unbalance said network, means driven by said nuid motor for changing the balance condition 4of said network, and means responsive to the balance or unbalance of said network for operatingsaid valve means.

10. A control lsystem, comprising in combination, a load device biased to a predetermined position, a pressure operated fluid motor for -driving said load against its bias, means for 20' supplying fluid under pressure, valve means i'or controlling the flow of fluid from said supply means to said fluid motor, leak means in said motor for permitting slow operation of said motor under the influence of said biased load device when said valve means is closed, a device having an electrical characteristic variable in accordance with a condition indicative of the need for operation of said load device. a normally balanced electrical network including said device, said device being operative upon a change in said condition to unbalance said network, means driven by said fluid motor for changing the balance 'condition of said network, second valve means for controlling the flow of fluid to said motor, said second valve means when open, permitting the ilow of iluid to said motor so as to drive said load rapidly in the direction of its bias, and means responsive to the balance condition of said network for operating said valve means, said balance responsive means opening said first valve means upon an unbalance of said network in a direction indicative of a need for movement of said load against its bias, and opening said second valve means only upon a relatively large unbalance of said network in a direction indicative o! a need for movement oi said load in the direction of its bias.

11. In combination, an aircraft, means defining a space within said aircraft, duct means for supplying air to said space, means for utilizing the motion of said aircraft to induce a flow oiair through said duct means, heater means,in said duct means, means for controlling the amount of heat transferred by said heater to the air flowing through said duct means, a first temperature responsive resistance element, means for mount ing said element in a position exposed to the temperature of the air in said space, a second temperature responsive resistance element, means for mounting said second element in said duct means on the up-stream side of said heater means, a third temperature responsive resistance element. means for mounting said third element in said duct means in the path oi air discharged from said heater means, said first element having a resistance at least ten times that of each of said second and third elements, a bridge circuit including all said elements in such a manner that a change in any oi the elements in a certain sense unbalances the bridge circuit in the same sense, and means responsive to unbalance of said bridge circuit tooperate said heat transfer controlling means.`

12. In combination, an aircraft, means defining a, space within said aircraft, duct means for supamaai? plying air to said space, means for utilizing the motion of said aircraft to induce a flow of air through said duct means, heater means in said duct means, means for controlling the amount of heat transferred by said heater to the air iiowing through said duct means, a first temperature responsive resistance element, means for mounting said element in a position exposed to the tempeature of the air in said space, a second temperature responsive resistance element, means for mounting said second element in said duct means on the up-stream side of said heater means, a third temperature responsive resistance element, means for mounting said third element in said duct means in the path of air discharged from said heater means, said first element having a resistance at least ten times and not more than fifty times that of each of said second and third elements, a bridge circuit including all said elements in such a manner that a change in any of the elements in a certain sense unbalances the bridge circuit in the same sense, and means responsive to unbalance of said bridge circuit to operate said heater controlling means.

13. Electrical temperature control apparatus for an aircraft, comprising in combination, heat producing means on said aircraft, means for utilizing the fiowof air caused by the motion of said aircraft to conduct air into heat exchange relation with said heat producing means, means for controlling the amount of heat transferred from said heat producing means to said air, reversible electrical motor means for operating said heat transfer control means, an electrical resistance element having an appreciable temperature coefficient of resistance and exposed to a temperature indicative of the need for operation of said heat transfer control means, means including said element for producing an alternating electrical signal of a phase dependent upon the direction in which said heat transfer control means needs to be operated, amplifier means for controlling the direction of operation of said motor means in accordance with the phase of said signal, said amplifier means having an input circuit and an output circuit, means coupling said signal producing means and said input circuit, means coupling said output circuit and said motor means, and means for supplying said signal producing means, said amplifier means and said motor means with alternating electrical energy of fixed frequency and predetermined phase relationships, said energy supplying means comprising a transformer having a primary winding and at least three secondary windings, means connecting one of said secondary windings to said signal producing means, means connecting a second of said secondary windings to said amplifier means, means connecting a third of said secondary windings and said motor means, and means including a battery and an inverter for supplying said primary winding with alternating electrical energy.

14. Electrical temperature control apparatus for an aircraft, comprising in combination, a heater on said aircraft, means for utilizing the flow of air caused by the motion of said aircraft to conduct air into heat exchange relation with said heater to warm said air, means for controlling the amount of heat transferred from said heat producing means to said air, reversible electrical motor means for operating said heat transfer control means, an electrical resistance elcment having an appreciable temperature coemcient of resistance and exposed to the temperature of the air warmed by said heater, means including said element for producing an alterhating electrical signal of a phase dependent upon the direction in which said heat transfer control means needs to be operated, amplifier means for controlling the direction of operation of said motor means in accordance with the phase of said signal, said amplifier means having an input circuit and an output circuit, means coupling said signal producing means and said input circuit, means coupling said output circuit and said motor means, and means for supplying said signal producing means, said amplifier means, and said motor means with alternating electrical energy of fixed frequency and predetermined phase relationships, said energy supplying means comprising a transformer having a primary winding and at least three secondary windings, means connecting one of said secondary windings to said signal producing means, means connecting a second of said secondary windings to said amplifier means, means connecting a third of said secondary windings and said motor means, and means including a battery and an inverter for supplying said primary Winding with alternating electrical energy.

15. Electrical temperature control apparatus for an aircraft, comprising in combination, an

engine on said aircraft, means for utilizing the flow of air caused by the motion of said aircraft to conduct air into heat exchange relation with said engine to cool said engine, damper means for controlling the iiow of air into heat exchange relation with said engine, reversible electrical motor means for operating said damper means, an electrical resistance element having an appreciable temperature coefficient of resistance and exposed to the temperature of said engine, means including said element for produc- `ing an alternating electrical signal of a phase dependent upon the direction in which said damper means needs to be operated to maintain the temperature of. said engine within a predetermined range, amplier means for controlling the direction of operation of said motor means in accordance with the phase of said signal, said amplifier means having an input circuit and an output circuit, means coupling said signalproducing means and said input circuit, means coupling said output circuit and said motor means, and means fOr Supplying said signal producing means, said amplifier means and said motor means with alternating electrical energy of nxed frequency and predetermined phase relationships, said energy supplying means comprising a transformer having a primary winding and at least three secondary windings, means connecting one of said secondary windings to said signal producing means, means connecting a second of said secondary windings to said amplifier means, means connecting a third of said secondary windings and said motor means, and means including a battery and an inverter for supplying said primary winding with alternating electrical energy.

16. Electrical control apparatus, comprising in combination, a load device to be controlled, reversible electrical motor means for controlling said load device, means including a device responsive to a condition indicative of the need for operation of said load device for producing an alternating electrical signal of a phase dependent upon the direction in which said load device needs to be operated, amplier means for controlling the direction of operation of said motor means in accordance with the phase of said signal, said amplier means having an input circuit and an output circuit, means coupling said signal producing means and said input circuit, means coupling said output circuit and said motor means, and means .for supplying said signal producing means, said amplifier means and said motor means with alternating electrical energy of ilxed frequency and predetermined phase relationships, saidenergy supplying means comprising a transformer having a primary winding and a plurality of secondary windings, means connecting one `oi said secondary windings to said signal producing means, means connecting a second of said secondary windings to said ampliiler means, means connecting a third of said secondary windings and said motor means, and means including a battery and an inverter for supplying said primary winding with alternating electrical energy.

i7. In combination. an aircraft, means defining a space within said aircraft, duct means for sup- DLViIiS air to said space, means for utilizing the motion of said aircraft to induce a flow of air through said duct means, heater means in said duct means, 'means for controlling the amount of heat transferred by said heater to the air flowing through said duct means, a iirst temperature responsive resistance element, means for mounting said element in a position exposed to the temperature of the air in said space, a second temperature responsive resistance element, means Y for mounting said second element in said duct means on the up-stream side of said heater means, a third temperature responsive resistance element, means for mounting said third element in said duct means in the path of air discharged from said heater means, one of said elements having a resistance at least ten times that of each oi' the other two elements, a bridge circuit including all said elements in such a manner that a change in any of said elements in a certain sense unbalances the bridge circuit in the same sense, and means responsive to unbalance of said bridge circuit to operate said heat transfer controlling means.

18. In electrical temperature control apparatus for an aircraft having heat producing means and means for utilizing the flow of air caused by motion oi said aircraft to conduct air into heat exchange relation with said heat producing means, means for modulatingly controlling the amount of heat transferred from said heat producing means to said air, an element having an appreciable temperature coefficient of resistance and exposed to a temperature within said aircraft indicative of the need for operation of said heat transfer control means, a normally substantially balanced electrical network including said element, an electronic amplifier having an input circuit and an output circuit, means for rebalanc. ing said network, electrical motor means for operating said heat transfer control means and said rebalancing means simultaneously, a ccntinuously conductive electrical connection be1 tween said network and said input circuit, and another continuously conductive electrical connection between said output circuit and said motor means.

19. In electrical temperature control apparatus for an aircraft having an engine and means for utilizing the ilow of air caused by motion of said aircraft to conduct air into heat exchange relation with said engine to cool the same, means for modulatingly controlling the flow of air into heat exchange relation with said engine, an element having an appreciable temperature coefficient oi" resistance and exposed to the temperature of said engine, a normally substantially balanced electrical network including said element, an electronic amplifier having an input circuit and an output circuit, means for rebalancing said network, eiectrical motor means for operating said air flow controlling means and said rebalancing means simultaneously, a continuously conductive electrical connection between said network and said input circuit, and another continuously conductive electrical connection between said output circuit and said motor means.

WILLIS H. GELE. 

